You just did, in this very post:
John says man can-and must be-righteous:
"Dear children, do not let anyone lead you astray. The one who does what is right is righteous, just as he is righteous. The one who does what is sinful is of the devil, because the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the devil’s work. No one who is born of God will continue to sin, because God’s seed remains in them; they cannot go on sinning, because they have been born of God. This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not God’s child, nor is anyone who does not love their brother and sister." 1 John 3:7-10
Depending on one's understanding of the doctrine of Sola fide, the following are the equivalent of reprieves from man's obligation to be rigtheous:
1) not sins, past, present, and future, are counted against a believer such that no future sin could ever separate one from God
and/or:
2) a believer is given a sort of make-believe vicarious strictly declared righteousness that precludes him form having to actually be righteous and live accordingly
So…to say that a believer is either forgiven of all unrighteousness past, prestm, and future, or that he's declared to be righteous when he is not-or both- in these views man need not actually be righteous in order to be seen as righteous-or justified- in God’s eyes.
The change in us that you speak of is part of our justification-and it's the reason we begin to do good and overcome sin. It's to become Christlike, to begin to love as He does, a gift of grace (Rom 5:5). Without works, "works of grace" prepared for us in advance such as those referred to in Eph 2:10, or the work of overcoming sin (Rom 8:12-14) there's no salvation.
"To those who by persistence in doing good seek glory, honor and immortality, He will give eternal life." Rom 2:7
"Make every effort to live in peace with everyone and to be holy; without holiness no one will see the Lord." Heb 12:14
"Therefore, brothers and sisters, we have an obligation—but it is not to the flesh, to live according to it. For if you live according to the flesh, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live. For those who are led by the Spirit of God are the children of God." Rom 8:12-14
To presume that this righteousness is guaranteed to "happen" to us apart from our willing participation or cooperation in this work of God's, or to presume that a believer can't turn permanently back away from God, is to ignore Scripture, and real-life experience. The battle of good over evil rages on within all believers even while they're now equipped to win it-but the outcome, as to how we did with all we've been given, will be weighed by the Just Judge at the end of the day. The Parable of the Talents or Bags of Gold sheds light on this matter.